One use for ion implanters is in doping silicon wafers to form semi-conductors. If the impurities used in doping the wafers can be ionized and formed into an ion beam, the ion beam can be used to dope the silicon wafers by causing the ion beam to impinge upon the wafers.
One problem experienced in ion implantation doping systems is the problem of wafer charging. As the ion beam is directed into contact with the wafer, the wafer charges as the positively-charged ions strike the wafer surface. The charging is often nonuniform and can create large electric fields at the wafer surface which can damage the wafer, making it unsuitable for use as a semi-conductor material.
In some prior art implantation systems, an electron shower device is used to neutralize the space charge of the ion beam. Existing electron shower devices utilize secondary electron emissions caused when an energetic electron strikes a metal surface. Low-energy electrons from the metal surface are either trapped in the ion beam or are directed to impact the wafer surface thereby directly neutralizing the wafer.
The current density of electrons obtained by secondary emissions from a metal surface is limited by the potential difference between the ion beam and the emitting surface. As the beam potential drops due to better neutralization, the secondary emission electron current that can be extracted from the emitting surface decreases. In the case of a charged ion beam directed onto an isolated wafer, the electron current must be equal to the ion beam current for the neutralizer to prevent wafer charging. If the beam potential is initially low, the wafer charges until the ion beam potential is large enough to extract a required amount of electron current from the secondary emission surface. A low potential ion beam does not mean wafer charging does not occur since the center of the beam is positive and the exterior negative due to concentrations of negative charge surrounding the ion beam. In practice, it has been found that the space charge at the metallic electron emitting surface is partially neutralized by slow ions from residual gas ionization along the ion beam path. As a result, higher electron currents than would be expected from theory can be extracted and, in fact, prior art techniques take advantage of residual gas pressures to provide low-energy electrons.
U.S. Pat. No. 4,804,837 to Farley discloses a beam neutralizing system having a source of high-energy electrons which are deflected back and forth through a neutralizing region containing an ionizable gas. As the high-energy electrons pass through the region, they ionize the gas providing low-energy electrons for beam neutralization. The disclosure of the '837 patent to Farley is incorporated herein by reference.
If an ion beam cannot be "sufficiently" neutralized, it tends to blow-up or expand due to the mutual repulsion of the positively-charged ions within the beam. To minimize the region of this blow-up, an electron barrier can be placed up-stream from the electron shower. A typical ion implantation chamber has a support that rotates silicon wafers through the ion beam along a circular path so that the ion beam encounters a wafer, then a wafer support held at ground potential, and then a next subsequent wafer, etc. This causes the ion beam potential to rapidly fluctuate as the wafers pass through the ion beam. Variations in this beam potential are reduced by placement of a constant potential aperture plate upstream from the region of beam neutralization.
Fluctuation in beam potential is reduced by placement of a negatively biased aperture that produces a potential minimum along the ion beam. Electrons from the beam neutralizer cannot penetrate this aperture.
The use of suppression apertures has two undesirable consequences. The aperture introduces a boundary condition causing a sharp divergence in beam potential. This can exacerbate the beam blow-up downstream from the suppression aperture. Additionally, the electric field in the region of the aperture can deflect electrons within the beam neutralizer downstream to the region of the implantation chamber. This has the undesirable result of producing a region of positive charge at a central core of the ion beam and a negative charge at the outer periphery of the ion beam. Stated another way, the ion beam is neutral in a broad sense, but at the wafer surface, a positive charge build-up occurs at the wafer center and a negative charge occurs around the outer circumference. This results in the creation of large, undesirable electric fields.
A stated goal of the '837 patent to Farley is to increase the time period electrons in the region of the ion beam encounter gas molecules thereby increasing the production of secondary electrons.